radar

Radar is a device to send and receive radio signals that bounces them off targets to obtain information about the targets. (The term radar was coined as an acronym for radio detection and ranging, but is now treated as a word, i.e., lower-case letters). Radar is used on planetary missions and has also been used by radar located on or orbiting Earth that can reach the Moon and inner solar system planets.

The most basic information provided by radar is the distance to the target, derived from the time interval from sending to receiving a change in the signal, and the direction of the target using a highly-directional transmitter and/or receiver, e.g., with a dish. Circular polarization radar offers additional information about characteristics of the target.

Radio with a frequency of tens of megahertz (10 MHz-2.6 GHz) is not fully stopped by the ground, and radar using its penetration (called ground-penetrating radar or GPR) can offer information about the structure of moons and planets. Varying materials, e.g., underground layers, vary in their transparency to radio and create partial reflections, analogous to optical effects, information useful to determining and/or verifying what is below the surface.

Analogous technology has been developed for other portions of the electromagnetic spectrum, such as lidar (aka LiDAR or LIDAR, meaning light imaging detection and ranging or laser imaging detection and ranging), which uses lasers, typically using infrared light.